Rate-determining step Knowledge-Based Download PDF

Questions

1. 1.

   Which statement best explains why the slowest elementary step in a multistep reaction mechanism governs the rate of the overall reaction?

   • A) It has the highest activation energy among the elementary steps, creating a kinetic bottleneck that restricts the maximum rate of product formation.
   • B) It is the step with the most negative standard Gibbs free energy change, thermodynamically driving the overall reaction to completion.
   • C) It involves the collision of the largest number of reactant molecules, requiring the greatest molecularity in the pathway.
   • D) It always occurs as the final elementary step in the mechanism, directly forming the isolated terminal products.
2. 2.

   When formulating an overall rate law from a mechanism where the rate-determining step consumes an intermediate, which species must be algebraically eliminated so that the rate law contains only measurable quantities?

   • A) The homogeneous catalyst
   • B) The reaction intermediate
   • C) The limiting reactant
   • D) The final product
3. 3.

   How does increasing the concentration of a reactant that participates exclusively in an elementary step occurring after the rate-determining step affect the initial rate of the overall reaction?

   • A) It increases the reaction rate proportionally because every species in the stoichiometric equation must appear in the rate law.
   • B) It decreases the reaction rate by rapidly depleting reaction intermediates required for the preceding slow step.
   • C) It exponentially increases the reaction rate by lowering the activation energy barrier of the entire mechanism.
   • D) It has no effect on the reaction rate because the overall rate is governed entirely by the steps up to and including the rate-determining step.
4. 4.

   A multistep chemical reaction proceeds via three consecutive elementary steps with activation energies of $45\text{ kJ/mol}$, $88\text{ kJ/mol}$, and $22\text{ kJ/mol}$, respectively. Assuming similar frequency factors, which step is the rate-determining step, and why does it govern the overall reaction rate?

   • A) Step 3, because its lowest activation energy allows intermediates to react most rapidly.
   • B) Step 1, because the initial collision between starting reactants sets the speed for all subsequent steps.
   • C) Step 2, because the largest activation energy results in the highest frequency of successful collisions.
   • D) Step 2, because having the highest activation energy makes it the slowest step that limits the overall reaction rate.
5. 5.

   A proposed two-step mechanism for the reaction between nitrogen dioxide and carbon monoxide consists of a slow first step, $\text{NO}_2\text{(g)} + \text{NO}_2\text{(g)} \rightarrow \text{NO}_3\text{(g)} + \text{NO}\text{(g)}$, followed by a fast second step, $\text{NO}_3\text{(g)} + \text{CO}\text{(g)} \rightarrow \text{NO}_2\text{(g)} + \text{CO}_2\text{(g)}$. Which rate law is consistent with this mechanism?

   • A) $\text{Rate} = k[\text{NO}_2][\text{CO}]$
   • B) $\text{Rate} = k[\text{NO}_2]^2[\text{CO}]$
   • C) $\text{Rate} = k[\text{NO}_3][\text{CO}]$
   • D) $\text{Rate} = k[\text{NO}_2]^2$